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High Voltage Bench Power Supply Design

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H713:
I was thinking about doing a crowbar, however I'm wondering if you had anything in mind in terms of implementation.

The first idea that comes to mind is to use a two-gang pot for the voltage adjust, the second of which forms an adjustable voltage divider. That along with a zener could be used with a beefy triac if the output voltage goes noticeably higher than it's supposed to. What I'm not a huge fan of about this is that it's putting a fair bit of trust in the wiper of a potentiometer.

The second idea is a fixed crowbar circuit for say 500V. If the adjustment is limited from 0-450V, and it goes up to say 500 (indicating a pass element short), then the crowbar would be triggered (again using a zener, but this time with a fixed voltage divider).

Attached is the world's ugliest schematic to try and illustrate my idea.
Is this what you had in mind?

Wolfgang:
You got the idea.

In the circuit below:

https://electronicprojectsforfun.wordpress.com/power-supplies/high-voltage-lab-power-supplies/a-1kv-50ma-linear-power-supply/

I have created an electronic "fuse" that switches off a preregulator output in case of *overcurrent*.
You could do something similar for *overvoltage".

001:
oh, man

it is long tale
no easy way to make it with transistors
check Heathkit units at ebay or rebuild Your own clone

Wolfgang:
Correct. Reliable transistor (or MOSFET) high voltage supplies are not as simple as they look.
But its an interesting subject, IMHO.

H713:
So I've drawn up a schematic with my idea as it stands now. I may choose to parallel a third transistor, though I do suspect that two would be more than enough. I still haven't worked out my current limiting circuit, so if anyone has an idea as to how to go about it I'm all ears.

One thing I am planning on implementing (though I didn't draw it in the schematic) is to have a "50-250" and a "250-450" voltage switch. In the 50-250 mode, it shorts across R1 and R5. This does several things. Firstly, it means I don't need to use a 500K pot for the voltage adjust (more precise setting), and it will limit the amount of power dissipated by the pass elements. This combined with a current limit (if I can come up with something suitable) and the crowbar circuit makes me feel much better about the reliability of the FETs.




Edit: Do NOT try to build this schematic as shown- Testing at a later date revealed that the FETs cannot meet their published specs and a pair of them in parallel will not be able to handle the load. More viable schematics are shown towards the end of page 3.

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